The present invention relates to a heat pump apparatus using a mixture of non-azeotropic refrigerants.
One prior art heat pump apparatus, which is known as an "inverter" system, is shown in FIG. 1 which comprises a compressor 40, a four-way valve 41, a heat exchanger 42 acting as a heat sink, an expansion device 43 and a heat exchanger 44 acting as a heat source, all of which are connected in a series circuit. The compressor is driven by a motor 45 which is controlled by a frequency converter 46 which converts the frequency of the mains supply 47 in response to manual commands. The rotational speed of the motor is controlled by varying the frequency of the current supplied from the frequency converter 46 in accordance with power demand. However, since the working fluid is of a single composition refrigerant, such as the type R22, and since the thermal transfer area of the heat exchangers 42 and 44 are constant, an increase in frequency causes the condensation temperature of the system to increase and the evaporation temperature to decrease. As illustrated in FIG. 2, when the operating frequency is low, the pressure-enthalpy cycle of the system follows a solid-line curve. However, when the frequency is high, the higher pressure of the system (condensation temperature) rises while the lower pressure (evaportion temperature) drops, resulting in a cycle indicated by a broken-line curve. Of primary concern is the shift from line a-b to line a'-b' (which represents the compression stroke of compressor 40) coupled with the increase in the condensation temperature results in a sharp increase at the outlet of the compressor, which could lead to the decomposition of the refrigerant and deterioration of thermal insulation. These factors present a significant increase in the load of frequency converter 46 at high frequency operation. In addition, refrigeration power output does not increase proportionally to the increase in frequency since the lower pressure drop causes the specific volume of the compressor's intake stroke to increase.
For the reasons given above, the upper limit of the variable frequency must be determined from the system's reliability point of view or determined by the maximum power delivered during startup period.
Another prior art heat pump apparatus, as disclosed in U.S. Pat. No. 2,938,362, comprises a rectifier in the main circuit of the apparatus and employs a mixture of non-azeotropic refrigerants. The rectifier controls the mixture ratio of the refrigerants so that the amount of the fluid circulating the main circuit is varied to meet desired power demand. The rectifier separates the mixture and stores the lower pressure refrigerant in a reservoir and circulates a fluid with a high content the higher pressure refrigerant through the main circuit during operation. A three-way valve is used to route the lower pressure refrigerant in the reservoir to be mixed with the fluid in the main circuit during standby periods to restore the mixture to the original ratio. However, the enrichment of the main circuit fluid is a process too slow to meet a sharp rise in power demand.
Our experiments show that for a given combination of higher and lower pressure refrigerants the amount of optimum working fluid in the main circuit increases with the content of the higher pressure refrigerant. Specifically, using refrigenrants R12 and R13B1 as low and high pressure refrigerants, respectively, zero content of R13B1 in the working fluid requires a total of 800 grams of refrigerant R12 for optimum condition, 50% content of R13B1 requires 950 grams of the mixture and 80% content results in the requirement of 1200 grams of the mixture. This is considered to arise from the fact that with increase in higher pressure refrigerant, the specific volume of the refrigerant gas decreases, causing the optimum volume of the working fluid in the refrigeration circuit to increase undesirably from the compressor's performance point of view. Furthermore, a substantial amount of refrigerant mixture must be stored during standby periods. In addition, an electric heater is required to heat the rectifier, increasing the total amount of energy. Since the rectifier is connected in a lower-pressure circuit, the working fluid empties the rectifier and enters the main circuit during cooling operation. As a result, the rectifier is inoperative during cooling operation and the amounts of working fluid required for both cooling and heating operations largely deviate from each other and thus it is impossible to operate the apparatus with an optimum amount of refrigerant.